The present invention relates generally to devices that recover, detect or demodulate signals, and more particularly, to devices that provider greater sensitivity and noise rejection for the detection of FM Doppler radar, geological or oceanographic sonar returns and for spread spectrum communication or hybrid envelope/exponent modulation systems.
Conventional FM detectors/demodulators suffer from a threshold phenomenon which limits detection sensitivity of exponentially modulated signals (also referred to as “angle modulated signals”). These detectors/demodulators typically utilize diode rectifiers and matched filters to recover the baseband information signals. However, such devices introduce non-linearities, e.g., noise cross products, that are the root cause of the threshold phenomenon.
As predicted by Claude Shannon, an FM demodulator is 1.77 dB more sensitive than a matched filter of equal bandwidth. Under this paradigm, the best that can be achieved by using the matched filter is an output signal-to-noise ratio (SNR) that is equal to the input carrier-to-noise ratio (CNR).
In particular, the current state-of-the-art in analog demodulators or detectors of exponentially modulated signals can be categorized into several broad classes. The first major class distinction considers the treatment of additive white Gaussian noise (AWGN). There are methods that convert or transform stationary AWGN to a parabolic noise density distribution and those that do not. Those that do convert AWGN to a parabolic distribution all have a CNR threshold limitation below which the conversion ceases. Foster-Seely, Travis and Ratio Detector types of exponential modulation demodulators, detectors or discriminators are the primary types that perform the conversion when operating at (C/KT) above the threshold limitation. See FIGS. 1A and 1B which depict a Travis FM discriminator and a Foster-Seeley discriminator, respectively.
Another general class of exponential modulation detectors utilize some form of product detection. This class of detectors do not convert AWGN to a parabolic noise power distribution. At best, they do not degrade the output detected signal-to-noise-ratio to a value worse than the input (CNR) or (C/KTB) where the input and output bands are equal. Among this type of detector are the Phase Locked Loop, the correlation detector. The Phase Locked Loop uses a voltage controlled oscillator (VCO) to provide a replica of the received signal. The phase error between the received signal and the VCO provides the signal that drives the VCO. It can have a threshold that is about 3 dB better that of the Foster-Seeley or Travis Demodulator. Other types of exponential modulation detectors are:                1. Pulse Counting Discriminator. This method uses a monostable multivibrator or other pulse generator that produces a pulse of constant amplitude and width each time the composite noise and signal voltage crosses a reference value. The output pulses are low pass filtered to reject the pulse repetition rate. Fluctuation of the average value of the LP Filter output is the baseband information. This type of demodulator has a high threshold and is seldom used.        2. The FMFB utilizes negative feed-back to compress the received spectrum prior to demodulation. This technique is effective for small information bandwidths and has been used to carry up to 600 telephone channels on a single FDMFM carrier. The threshold improvement is of the order of 3 dB.        3. I/Q Demodulators. This class of demodulator requires a high degree of synchronization with the transmitted signal. The received signal is broken down into In Phase and Quadrature components. The multipliers or mixers used to perform the conversion do not transform flat input noise density to parabolic and so are limited to matched filter performance. However, there is no threshold if the band can be made small enough. Using this type of detector with a 1 Hz bandwidth the signal and noise can be sampled and stored. Multiple samples can be processed to effectively decrease the bandwidth and increase (SNR). This technique is used to detect weak Doppler RADAR returns.        
Therefore, in view of the foregoing, all of these conventional demodulators fail to address the CNR threshold and, as a result, at or below this threshold the output signal is pure noise. Furthermore, because these configurations are demodulators, they do not operate as filters and consequently these demodulators cannot be cascaded.
Thus, there remains a need to overcome this threshold phenomenon by using filtering techniques which permit the cascading of stages thereof, that improves the SNR and which eliminates the need to utilize complex techniques to result in improved performance and design simplification.
All references cited herein are incorporated herein by reference in their entireties.